Permanent magnet employing an adjustable shunt internally of the permanent magnet structure



3,460,083 E SHUNT INTERNALLY UC'IURE Aug. 5, 1969 V BL ET STR 1967 U JN NDG2 AAl M N 8 AT n J uu .GE NN w 1 R i F PERMANENT MAGNET EMPLOYI OF THE PERMA INVENTOR. RICHARD L.JOHNSON My} 6a ATTORNEY United States Patent O 3,460,083 PERMANENT MAGNET EMPLOYING AN ADJUST- ABLE SHUNT INTERNALLY OF THE PERMA- NENT MAGNET STRUCTURE Richard L. Johnson, Menlo Park, Ca]if., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed June 12, 1967, Ser. No. 645,397 Int. Cl. H01f 3/12 US. Cl. 335-297 6 Claims ABSTRACT OF THE DISCLOSURE A high field permanent magnet apparatus is disclosed which is suitable for gyromagnetic resonance spectroscopy. The magnet includes a pair of coaxially disposed axially polarized permanent magnets spaced apart to define a high field gap. The permanent magnets are enclosed by a surrounding magnetic yoke structure serving to shield the magnetic gap. At least one of the permanent magnets is hollow on its axis to receive an axially movable magnetic shunt for varying the intensity of the field in the gap of the magnet. In one embodiment, the hollow magnet is held to the yoke by a non-magnetic tube axially disposed of the magnet. An axially expandable and contractable magnetic shunt is disposed in the tube for varying the field intensity in the gap.

DESCRIPTION OF THE PRIOR ART Heretofore, it has been proposed to vary the intensity of the magnetic field of closed yoke permanent magnets by varying the reluctance of the enclosing yoke structure. The prior field adjusting apparatus involved a relatively large section of the yoke which was moved in and out of the yoke by means of a relatively large jack screw device bridging across the end of the magnet. Such a device was relatively large and substantially increased the overall length of the magnet and generally added to the problems of thermal and magnetic shielding of the magnet.

Also, it has been proposed to cancel gradients in the gap of an electromagnet by means of an axially translatable ferromagnetic member, movable within an axial bore in one or both of the coaxially aligned magnetic cores of the electromagnet. Such devices are described and claimed in US. Patents 3,182,231 and 3,223,897 issued May 4, 1965 and Dec. 14, 1965, respectively. In the electromagnet, the ferromagnetic member does not serve to shunt the magnetic flux back on the magnet itself to vary the total field intensity in the gap, rather it serves to redistribute the magnetic flux in the gap of the electromagnet by operating on the flux distribution in the pole structures at the back face of the pole caps for removing certain gradients in the gap. Substantial changes in the total field intensity of the gap are to be avoided in such devices since the object is to vary the gradients without changing the homogeneous component of the field.

SUM-MARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved permanent magnet apparatus.

One feature of the present invention is the provision, in a permanent magnet having a pair of coaxially disposed axially spaced permanent magnets, of a movable magnetic shunt disposed within at least one of the permanent magnets for variably controlling the effective strength of the shunted magnet and, thus, of the field in the gap of the magnet, whereby the adjustable shunt 3,460,083 Patented Aug. 5, 1969 ice does not appreciably add to the size and complexity of the magnet and its shields.

Another feature of the present invention is the same as the preceding feature including a tube extending along the axis of the shunted permanent magnet for holding the permanent magnet to its yoke structure, and wherein the movable shunt is disposed inside the holding tube.

Anoher feature of the present invention is the same as any one or more of the preceding features wherein the magnetic shunt is axially expandable and contractable in length for varying the magnetic field intensity.

Other features and advantages of the present invention will become apparent upon a perusal of the following specifications taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a longitudinal sectional view of a permanent magnet incorporating features of the present invention, and

FIG. 2 is an enlarged detail view of a portion of the structure of FIG. 1 delineated by line 22.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown the magnet 1 of the present invention. The magnet 1 includes a pair of coaxially aligned permanent magnet structures 2 and 3 enclosed within a coaxial generally egg-shaped magnetic yoke 4, as of soft iron. The magnet structures 2 and 3 are fixedly held to the ends of the yoke 4 via a pair of axially directed non-magnetic tubes 5, as of non-magnetic stainless steel, as more fully described below. The yoke structure 4 includes two bowl-shaped sections which are joined together at their lips by a circumferentially directed joint 6. A hole 7 is provided in the yoke 4 to permit access to a magnetic gap 8 defined by the space between the inner ends of the permanent magnet structures 2 and 3.

The permanent magnet structures 2 and 3 each include a stack of three disk-shaped permanent magnets 11, 12, and 13 as of Alnico V-7 polarized in aiding magnetic relation, as indicated, to produce a pair of near poles of opposite polarity and a pair of remote poles of opposite polarity. The remote poles of opposite polarity are interconnected by the low reluctance magnetic yoke 4. A pair of mounting plates 14 as of soft iron are affixed to the support tube 5 at the end of the stack of magnets 11, 12, and 13 and a pair of pole caps 15 are mounted to the mounting plates 14, as by a plurality of screws disposed about the periphery of the pole caps 15. The magnet gap 8 is defined by the space between the pole caps 15. Gap 8 is about 3 inches in diameter and about a half an inch wide. The magnetic field H in the gap 8 has an intensity of about 14.5 kg.

A pair of magnetizing coils 16 are coaxially disposed of the permanent magnet structures 2 and 3 and are wound on cylindrical coil forms 17. The coils 16 are initially energized with a sequence of high current pulses to magnetize the permanent magnets 11, 12, and 13-. After the permanent magnets are magnetized, the coils 16 may be used to shift the magnetic in the gap 8.

Referring now to FIG. 2, the magnet holding tube 5 with its internal variable shunt structure is shown in greater detail. The inner end of the holding tube 5 is closed off by a threaded plug 18, as of non-magnetic stainless steel. The plug is screwed into the end of the tube 5 and also is screwed into a tapped bore 19 in the mounting plate 14. The outer end of the holding tube 5 is externally threaded to mate with a nut 21 which is tightened down over the tube 5 to slightly pull and hold the magnet stack against the end of the yoke 4.

A ferromagnetic tube 22 as of cold rolled steel, which forms part of the variable magnetic shunt structure is inserted within the holding tube 5. The internal tube 22 is externally threaded at its outer end to mate with internal threads 23 at the end of the holding tube 5. The internal tube 22 has an axial length about half that of the holding tube 5. The internal tube 22 is internally threaded to mate with external threads on a magnetic shunt rod 24, as of diameter cold rolled steel. In a typical example, the stainless steel holding tube 5 is 9 long, 0.75" O.D*., and has a 0.156" wall thickness; steel shunt tube 22 is 4.5" long, 0.5" CD. and has a 0.109" wall thickness; and steel shunt rod 24 is 4.5" long and 0D.

In operation, the magnets 11, 12, and 13 are energized by coils 16 with the variable shunting members 22 and 24 fully extended to provide the maximum amount of shunting efi'ect. The shunting members 22 and 24 serve to shunt a variable fraction of the magnetic flux of the permanent magnets back through the center bore in the permanent magnets 11, 12, 13. Typically, for the aforecited dimensions, the magnetic field in the gap will be nominally 14.1 kg. As the magnet 1 ages, the magnets 11, 12, and 13 will lose some of their magnetization causing the field to drift to a lower intensity. The field is restored to its initial intensity by retracting the shunting rod 24 a sufficient extent to reduce the shunting effect on the magnet. The magnet 1 may include a set of shunting members 22 and 24 in one or both of the magnet structures 2 and 3. With only one magnet shunting set, 22 and 24, the field may be increased by 30 gauss at 14.1 kg., with two sets, a total adjustment of about 60 gauss is obtainable. Adjustments of the magnet shunts do not adversely afieot the homogeneity of the magnetic field in the gap. Adjustment of the shunts is readily obtained via small axially aligned access holes, not shown, in the surrounding oven and thermal and magnetic shields. It is also found that adjustments of the field intensity by the shunt members 22 and 24 are reversible.

Since many changes could be made in the above construction and many apparenty widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a permanent magnet apparatus, means forming first and second coaxially disposed permanent magnet structures, said permanent magnet structures being axial- 1y spaced apart and axially magnetized to provide a pair of axially spaced near poles of opposite polarity, defining a magnetic gap therebetween, and a pair of remote poles of opposite polarity, means forming a magnetically permeable yoke structure interconnecting said pair of remote poles of opposite polarity to provide a low reluctance flux return between said magnet structure, the improvement comprising, means forming a magnetic shunt axially movable within said first permanent magnet structure for shunting a variable fraction of the magnetic field of said first permanent magnet structure back through the interior of the magnet to adjust the field intensity of the gap, and said magnetic shunt including a magnetic permeable structure which is expandable and contractable in the axial direction for adjusting its shunting effect on said first permanent magnet.

2. The apparatus of claim 1 including, means forming a tube coaxially disposed of and within said first permanent magnet structure for holding said first magnet structure to said yoke structure, and wherein said magnetic shunt is disposed inside of said holding tube.

3. The apparatus of claim 2 wherein said axially expandable shunt includes means forming a second tube, said second tube being made of a ferromagnetic material, said second tube being disposed within'said first holding tube and extending only partially the length of said holding tube for only partially shunting said magnet structure, and said shunt including a shunting member being axially movable in and out of said second tube for varying the magnetic field intensity.

4. The apparatus of claim 3 wherein said second tube which is made of ferromagnetic material is internally threaded to mate with external threads on said movable shunting member.

5. The apparatus of claim 2 wherein said holding tube is made of a non-ferromagnetic material to prevent magnetic shielding of said movable shunt.

6. The apparatus of claim 2 including means forming a holding tube coaxially disposed of and within said second permanent magnet structure for holding said second permanent magnet structure to said yoke structure, and means forming a second axially expandable ferromagnetic shunt member axially movable within said second holding tube for shunting a variable fraction of the magnetic field of said second permanent magnet structure to vari ably control the field intensity in the magnetic gap.

References Cited UNITED STATES PATENTS 3,018,422 l/196r2 Seaton 335-298 3,187,237 6/1965 Craig et a1 335-301 3,325,757 6/1967 Gang 335-306 XR 3,325,758 6/ 1967 Cook 335-306 XR GEORGE HARRIS, Primary Examiner U.S. Cl. X.R. 

